The present invention relates to radial piston machines in general, of the type operating with hydraulic or pneumatic fluid, and more particularly to a novel piston shoe which is used in such radial piston machines.
Radial piston machines are already well known, for instance from my own prior U.S. Pat. Nos. 3,223,046, 3,277,834, and 3,304,883. These types of piston machines are suitable as motors, as pumps, compressors, and the like, and have a component which is provided with an inwardly directed annular control face within the confines of which a rotor turns, the rotor being provided with substantially radial piston bores in each of which a piston is reciprocable. The outer end of the piston carries a piston shoe by means of which it is in engagement with the control face.
The piston shoes disclosed in my prior U.S. patents mentioned above, are already provided with hydrostatic bearings by being formed, in their outwardly directed surfaces which face the control face, with depressions which communicate with bores in the piston shoe and the associated piston, and via these bores with pressure medium in the cylinder in which the piston reciprocates. Thus, the pressure medium can establish a hydrostatic pressure field between the outwardly directed surface of the piston shoe and the control face, the purpose being to reduce the friction between this surface and the control face and to make it possible to operate radial piston machines provided with such hydrostatic bearings at higher operating pressures.
My continuing investigations have shown, however, that these prior art constructions have certain disadvantages.
In particular, the depressions for forming the hydrostatic bearings were in form of blind bores formed in the outwardly directed surface of the piston shoe and communicating with a fluid supply passage in the latter. There was no means for precisely defining the boundaries of the hydrostatic pressure field. A further difficulty arose from the fact that these bores were located approximately centrally of the outwardly directed side faces of the piston shoes. These two factors brought with them disadvantages which become apparent only over a period of time, and only as the requirements made of radial piston machines in terms of higher operating pressures and greater speeds of rotation of the rotor began to increase. In particular, the arrangement of the bores wherein the hydrostatic bearings developed, at the center of the piston shoe contact surfaces, caused an "aging effect" to take place, in the piston shoe over a period of time, with the result that the outer ends of the piston shoe tended to bend radially inwardly (towards the rotor) by some thousands or even hundreds of a millimeter. This resulted in increased leakage of fluid outwardly from the hydrostatic pressure field, and consequently in an increased friction between the piston shoe and the control face; both of these factors increased even further, the higher the operating pressure of the radial piston machine became. It was found that these two factors influenced the effectiveness of the machine to such an extent that in the case of certain piston shoes the operational effectiveness of the machine dropped below 85%.
Moreover, the fact that a simple blind bore was formed in the outer piston shoe guide face also facilitated fluid leakage and increased friction.
It was by no means evident that the aforementioned problems were caused by the location and the manner of forming the bores wherein the hydrostatic pressure field developed. Rather, the reduced operating effectiveness of radial piston machines was generally considered a result of a defect of other components which cooperated with the radial piston machines, for instance electromotors, combustion engines, or gas turbines used to drive the radial piston machines.